This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
The A-interface is defined in 3GPP as the terrestrial interface between the MSC node and the BSS radio network. Today TDM is the only defined bearer technology for user plane connection defined in the A-interface. The A-interface is defined in 3GPP technical specification 48.001 (3GPP TS 48.001 Base Station System—Mobile-services Switching Centre (BSS—MSC) interface; General aspects) and the references specified therein.
Handover procedures are defined in 3GPP technical specification 23.009 (3GPP TS 23.009 Handover Procedures). The technical specification assumes an A-interface as defined in the 3GPP standard (3GPP TS 48.001 Base Station System—Mobile-services Switching Centre (BSS—MSC) interface; General aspects).
FIG. 1 shows the procedure for an intra MSC, inter BSC, GSM to GSM handover procedure taken from 3GPP TS 23.009 Handover Procedures. This procedure is chosen as an example because it is used below to describe the concept of the new invention.
In regard to problems with existing solutions, currently the effort is taken to enhance the current standardized A-interface and to support the A-interface as well for IP used as bearer for the user plane. Although in the further disclosed, reference is made to IP technology as a further supported technology, the invention is not limited thereto but encompasses any differing bearer technology. It is assumed that during a network migration from the currently standardized A-interface (AoTDM) to the enhanced A-interface (AoIP) the two bearer technologies defined for the user plane are used in parallel. This may not be needed in all migration scenarios but it is seen as a likely scenario for many operators.
Applying the current 3GPP handover procedure (3GPP TS 23.009 Handover Procedures) in such a scenario—parallel use of TDM and IP as user plane bearer—results in the following issue: When the MSC request channel assignment from the target BSC then the MSC does not know if this BSC can establish the terrestrial interface using IP as bearer type. The BSC has the final decision on the bearer type because of the following rules:                In the BSC the decision for a specific bearer on the terrestrial interface may depend on the selected speech coder version for the A-interface.        The BSC has always the final decision regarding the radio codec and the same codec should be used on the terrestrial interface as on the radio link to avoid additional transcoding.        
The MSC has to seize bearer resources for the A-interface user plane (terrestrial interface) before it sends BSSMAP Handover Request message to the target BSC. If the target BSC cannot support TDM bearer for the terrestrial interface then BSC has to reject the call. MSC could repeat the BSSMAP Handover Request message using in the new request IP bearer for the terrestrial interface. This handling would require additional signaling on the A-interface and would extend the handover time.
One simple solution to avoid the repetition of the handover request message would be to provide some configuration in MSC about the BSC capability. However, this solution has the following disadvantages:                It is static and cannot provide BSC capability for a specific call        It is error-prone due to manual interaction        
Another sub-optimal solution would be that the BSC always accepts the bearer type offered from MSC in the handover request. If it cannot use the codec selected on the radio link on the terrestrial interface, then BSC should use another offered codec type. This solution has the following disadvantages:                The operator has to provide transcoder resources in the BSC to cover the described scenario        Inserting a transcoder in the BSC and using compressed speech codec on the terrestrial interface reduces speech quality and increases the delay in the speech path        